Conventionally, technologies by which a heat exchanger unit with a compact configuration is obtained by interconnecting multiple small plate heat exchangers in series have been known (e.g., see Japanese Patent Publication Nos. 2000-180076, 2000-356483 and 2005-337688).
In a heat exchanger unit with this configuration, particularly when this heat exchanger unit functions as an evaporator, the state of the refrigerant that flows therethrough changes such that the ratio of gas regions in the refrigerant gradually becomes higher as the refrigerant flows into the plate heat exchangers in a gas-liquid mixed state and travels to the plate heat exchanger on the downstream side.
Further, in the individual plate heat exchangers, the more liquid regions there are in the refrigerant, the more the refrigerant distribution performance with respect to each of the refrigerant flow paths inside drops, and sites where the heat exchange efficiency is high and sites where the heat exchange efficiency is low arise in the plate heat exchangers, so that the overall heat exchange efficiency drops.
Consequently, in order to improve the coefficient of performance of the heat exchanger unit overall, it is necessary to consider both the pressure loss and the refrigerant distribution performance in each of the plate heat exchangers.
From this standpoint, in patent citation 3, there is proposed a technology where a heat exchanger with the required capacity is configured by interconnecting two small plate heat exchangers in series. The heat exchanger is configured such that a distribution pipe to each of the refrigerant flow paths is disposed in the plate heat exchanger on the upstream side to ensure refrigerant distributivity, because there are many liquid regions in the refrigerant flowing into the plate heat exchanger on the upstream side and the distributivity of the refrigerant to each of the refrigerant flow paths is poor, and such that a distribution pipe is not disposed in the plate heat exchanger on the downstream side because there are many gas regions in the refrigerant flowing into the plate heat exchanger on the downstream side and the distributivity of the refrigerant is good.
According to this, in the plate heat exchanger on the upstream side, the pressure loss becomes large because of the presence of the distribution pipe, but the distributivity of the refrigerant improves because of the gas-liquid mixing action in the distribution pipe. Further, in the plate heat exchanger on the downstream side, the distributivity of the refrigerant is good because there are many gas regions, and the pressure loss is also small because there is no distribution pipe. Because of these synergistic effects, the coefficient of performance of the heat exchanger overall is improved.